Representative fractionation methods of fats known in the art are categorized into a solvent fractionation method for fractionating a raw material fat with an organic solvent such as acetone or hexane, and a dry fractionation method without using any solvents. While a dry fractionation method is more preferable than a solvent fractionation method in terms of the cost and safety, it is a problem of this method that separation performance between a crystalline fraction and a liquid fraction is low as compared with a solvent fractionation method and a considerable liquid fraction gets mixed with a crystalline fraction. Accordingly, dry fractionation has been seldom used as a fractionation method in such a stage for obtaining a hard butter that molecular species have been increased after treatment in processing steps such as hydrogenation and interesterification, though a dry fractionation method may be employed in fractionation of a crude fat. On the other hand, while a solvent fractionation method has high solid/liquid separation performance, according to the present inventors' experience, there arise problems of a non-recovered crystalline fraction and mixing of a liquid fraction with respect to fractionation of an isomerization hydrogenated containing similar melting point fractions or similar molecular species, and the method is still insufficient.
Another method known in the art is removal of high-melting and low-melting point fractions by two-stage fractionation steps to obtain a medium-melting point fraction. In particular, a medium-melting point fraction obtained by fractionating an isomerization hydrogenated fat of, for example, palm oil is used for improving physical properties of chocolates as a substitute of cocoa butter. Fats used for chocolates are called as hard butters and include laurin fats in addition to cocoa butter and isomerization hydrogenated fats. A medium-melting point fraction obtained by solvent fractionation from an isomerization hydrogenated fat usually has poor melting properties in the mouth and no sharp melting curve as compared with cocoa butter and laurin fats. In addition, a medium-melting point fraction has such a defect that it separates into a high-melting point fraction and a low-melting point fraction with time to readily worsen melting properties and to tend to generate low temperature blooming. According to the inventors' experience, this is caused by insufficient fractionation due to, for example, the effect of incorporation of liquid oils into crystals during crystallization, separation performance at the time of filtration, and mutual solubilization of triglycerides having similar melting points. This is supposed to be due to time-dependent changes of a fat from a mixed crystal state at the beginning of crystallization, in which molecules are uniformly dispersed, to an eutectic state, in which molecules having similar melting points are aggregated together because of mixing of a high-melting point fraction and a low-melting point fraction into a medium-melting point fraction to cause worsening of melting properties and softening.
Particularly, in the production of a hard butter mainly composed of G2U, it has been difficult to efficiently remove components having higher melting point than G2U, typically GGG (G represents a saturated or trans-fatty acid residue) and GG-DG (diglyceride having two saturated or trans-fatty acid residues). When a hard butter having a large content of GGG is used for a chocolate, melting properties of the chocolate in the mouth become poor. Further, when a hard butter having a large content of GG-DG is used for a chocolate, there arise problems of crystallinity (cooling curve) and tempering properties of the chocolate.
Methods for enhancing solid/liquid separation efficiency in dry fractionation using an adsorbent and a crystallinity improving agent have been disclosed in JP 4-154897 A and JP 10-511420 A for improving the above drawbacks. However, these methods also have such problems as loss of fat components due to the adsorbent treatment and difficulty in removal of the crystallinity improving agent. Further, JP 4-306296 A discloses a method for improving solid/liquid separation efficiency by warming a solid fatty acid faction containing liquid fatty acids at a temperature below the melting point of the fraction to cause perspiration of liquid components. However, this method is different from the method of the present invention in that the disclosed method is a fractionation method using a humectant, i.e., a fractionation method using a solvent, and in that the disclosed method is not drawn to separation of similar triglycerides. Furthermore, JP 6-234695 A discloses a method for obtaining high purity crystals by pressing crystals after pressure crystallization, followed by causing perspiration by depressurization, and further pressing to enhance solid/liquid separation efficiency. This method is also different from the method of the present invention because it relates to a purification method of fatty acids.